Medical personnel routinely slap an oxygen mask on people struggling to breathe as well as on stroke victims left oxygen-deficient in some parts of their brains. Until recently, doctors believed this was the fastest and most effective way to deliver oxygen to needy lung or brain tissue.

But pure oxygen causes rapid breathing, meaning that as it is pumped into the lungs, more carbon dioxide is exhaled, "and that makes the blood vessels much smaller," says Ronald Harper, a neurobiology professor at the University of California, Los Angeles, and senior author of the study published today in PLoS Medicine. The shrunken vessels "cannot deliver as much blood—or the oxygen that's in the blood—to the brain" or the heart.

Harper and his colleagues used functional magnetic resonance imaging (fMRI) to trace the effects of pure oxygen on the brains of 14 healthy children, ages eight to 15 years. In addition to constricting blood flow, administering pure oxygen caused some areas of the subjects' brains to go haywire: the hippocampus (buried deep in the midbrain), the insula (located in the brain's center), and the cingulate cortex, part of the its outermost surface. Harper says that these regions are generally linked to learning, memory and emotions, but also regulate pain, stress and blood pressure. In addition, he says, they signal the hypothalamus to kick into action. The hypothalamus is the brain's master gland, regulating everything from the body's temperature and heart rate to its internal clock (by serving as a link between the nervous system and various glands throughout the body).

"Those brain areas which influence the hypothalamus, when you give 100 percent oxygen, they turn on like crazy," Harper says. "They begin driving the hypothalamus very hard." The action triggers the gland to flood the blood with catecholamines: hormones (such as adrenaline and norepinephrine) and neurotransmitters, like dopamine, all of which feed into the pathway that controls contraction of the heart muscle. This chemical dump disrupts the heart rate, causing a reduction in blood flow and, hence, less oxygen being delivered to cells.

The good news: researchers discovered that the negative effects can be avoided if the oxygen is mixed with as little as 5 percent carbon dioxide before being administered. The combo neither triggered significant changes in any of the aforementioned brain regions nor disturbed the hypothalamus.

"The downsides of continuing to overlook the dangers of this [type of treatment] may [result in] a considerable amount of harm to patients," says researcher Joe Fisher, a professor of medicine at the University of Toronto, who in 2002 suggested that giving pure oxygen to patients poisoned by carbon monoxide might exacerbate rather than limit or reverse damage. "You would think it's like chicken soup to the soul," he says about the traditional wisdom of administering oxygen. "[But], we saw a mechanism that could lead to additional damage."

Based on the brain-imaging findings, "It's hard to imagine a situation where you would want to give oxygen without carbon dioxide," says the study's lead author, Paul Macey, an assistant neurobiology researcher at U.C.L.A. Harper adds that concentrated feeds of room air, containing only 21 percent oxygen, have proved effective in resuscitating newborns who have trouble breathing and turn blue (from a lack of oxygen). "I think that any administration of high levels of oxygen should be reviewed," he cautions, "to determine whether it is necessary," given the dangers.